Optimal orthodontic treatment could be possible when a orthodontist can predict and control tooth movement by applying a planned force system to the dentition.
The moment to force(M/F) ratio at the bracket has been shown to be a primary determinate of the pattern of tooth movement. As various M/F ratios are applied to the bracket on the tooth crown, strain distribution in periodontium can be changed, and the center of rotation in tooth movement can be determined. It is, therefore, so important in clinical orthodontics to predict the strain distribution in a force system of a M/F ratio.
The purposes of this study were to analyze the strain distribution in orthodontic force system by strain gauge attached to root surfaces, to evaluate the pattern of tooth movement of canine and fast molar, and to investigate the way of anchorage control of first molar during canine retraction into an extraction site with or without continuous arch wire. For this study, an experimental upper arch model with premolar extraction was constructed with photoelastic resin, where 8 strain gauges were attached on the root surface of canine, and 15 strain gauges were attached on upper first molar.
Forces from Nitinol closed coil spring for canine retraction were 200g and 400g, respectively. Three types of maxillary canine retraction and six types of anchorage reinforcement on maxillary first molar were compared.
The results were as follows.
1. Strain gauges attached on root surfaces were found useful for evaluation of the pattern of strain distribution in orthodontic force system.
2. Compression strain was highly concentrated at the distal root surface of the canine and the menial root surface of the 1st molar during canine retraction without continuous arch wire, and the difference was great between apical and cervical area.
3. The concentration of compression strain was decreased with application of continuous arch wire, and the
difference was reduced between apical and cervical area.
4. The concentration of compression strain was greater with a heavy force than with a light one, and the difference became great between apical and cervical area.
5. The strains in the first molar were even distributed with the reinforcement of anchorage in the first molar.
|